Floating point exception

It is already confusing enough that “floating point exception” may mean “division by zero” in integral arithmetic. It turns out it can also mean “overflow” in some cases, as in the following program (it’s difficult in C, so I had to use assembly):

#include <asm/unistd.h>
.code:
.globl _start
_start:
    mov $1, %eax
    mov $1, %edx
    div %eax
    mov $__NR_exit_group, %eax
    int $0x80

(Type “gcc -m32 -nostdlib a.S” to compile and link.)

In this program, EDX:EAX (0x100000001) divided by ECX (0x1) cannot be represented in 32-bit integer and thus it is an overflow. X86 CPUs raise a “division by zero” interruption (int 0) in such cases, and “division by zero” is displayed as “floating point exception” in Linux…

PS. The same assembly program in Intel style:

.code
.startup
    MOV EAX,1
    MOV EDX, 1
    DIV EAX

    MOV EAX, __NR_exit_group
    INT 80H

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Tags: assembly, Linux